1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method, a computer program, and a recording medium for allowing gradation characteristics of an image output apparatus to be gripped in a digital copying machine, a facsimile, a printer, and the like that form a gradation image, and for allowing gradation correction parameters suitable for the gradation characteristics of the image output apparatus to be generated.
2. Description of the Related Art
In an image output apparatus, even if the same image data is output, the density may change over time. Therefore, to suppress the density fluctuation occurring over time, a conventional image output apparatus has been designed to output predetermined patches, and a scanner or the like is used to read the patches to get a grip on gradation characteristics of the image output apparatus at that time and to generate gradation correction parameters.
However, an image output apparatus may output the same image data in different densities depending on the position of an image. In an electrophotographic image output apparatus, for example, such a change could occur due to the member quality or the precision of an assembly, such as eccentricity of a photosensitive element or a transfer roller, or a fluctuation in the space between the photosensitive element and a developing sleeve at a position along the direction of the rotating shaft of the photosensitive element.
In response to this issue, Japanese Patent Application Laid-open No. 2008-209436 and Japanese Patent Application Laid-open No. 2009-38734 suggest apparatuses that generate a plurality of patches having the same density and the same gradation value, measure the density values of the patches, and take an average of the measured density values to generate gradation correction parameters for suppressing the effect of the density fluctuation occurring depending on positions of images, while generating density correction parameters for suppressing a density fluctuation occurring over time as well.
For example, according to the disclosure in Japanese Patent Application Laid-open No. 2008-209436, an image forming system arranges a plurality of color measurement patterns in a direction in which the fluctuation of printing characteristics is expected to be large. Furthermore, as a method for determining a chart for suppressing the effect of in-plane unevenness by outputting repetitive patterns and obtaining the average of patches of, the same color, Japanese Patent Application Laid-open No. 2009-38734 discloses a method for selecting a pattern to be output by which the colors of patches included in a patch chart, i.e., an arrangement of patches of the same colors, are measured, the measurements are averaged as a reference value, and a repetitive pattern closest to the reference value is selected.
In generating gradation correction parameters, a precision degradation resulting from the in-plane unevenness caused by an image output apparatus may be suppressed by arranging the same patches at different positions, as disclosed in Japanese Patent Application Laid-open No. 2008-209436 and Japanese Patent Application Laid-open No. 2009-38734.
However, the technology disclosed in Japanese Patent Application Laid-open No. 2008-209436 takes the direction in which the patterns are positioned with respect each other into consideration, but does not take the distance between the patterns into consideration. Thus, the effect of the density fluctuation caused by an image output apparatus may not be suppressed sufficiently. Furthermore, in the technology according to Japanese Patent Application Laid-open No. 2009-38734, the average of the measurements of the patches arranged at different positions is used as a reference. However, when an average is used as a reference, it may not be always possible to obtain a repetitive pattern suitable for suppressing the effect of the in-plane unevenness for the following reasons.
For example, if the main cause of the density fluctuation is eccentricity of the photosensitive element, the cycle of the density fluctuation along the rotating direction of the photosensitive element is largely dependent on the circumferential length of the photosensitive element. To generate proper gradation correction parameters, it is necessary to suppress the effect of density unevenness resulting from the periodic component. For example, assuming that the density fluctuation is a sine-wave fluctuation with reference to zero density in the cycle of the circumferential length of the photosensitive element, the zero density must be identified highly precisely.
Assuming that five patches are output at equally spaced five points A to E on a piece of paper and an average density is calculated based on the points A to E, and the circumferential length of the photosensitive element is equal to the distance between A and E as illustrated in FIG. 8A, the densities at the certain phase angle of the photosensitive element corresponding to the positions A and E affect the average density largely. In other words, as illustrated in FIG. 8B, the average density (dotted line) of the five points, which are the average of the densities a to e of the patches arranged at A to E, is generally not the same as the zero density.